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  • Tools #2

    Volt/Ohm meters

    • Authored by Wes Vann, last revised on April13, 1997​

      • This page is intended to be basic in nature! It's almost a standardto use water in explaining how electricity works, and that's what I'llbe doing. The place where the water comparison falls apart is that yourgarden hose doesn't need a complete circuit to have the water flow.

        PLEASE NOTE; Although most of the voltage found in a car can'tcause physical harm, it is possible to burn up the wiring or even starta fire with a careless spark!


        In order for electricity to work, it needs a complete circuit.It has to be able to not only leave the battery, it has to have a pathback to it! Normally, the last part of that path is through the metal ofthe body and this is called "ground". All references to a voltageare in relationship to this ground.

        Volts; This is the force behind the electricity. Or the "pressure"at the faucet.

        Amps; This is how much electricity flows. Or gallons per minuteof water through the hose.

        Ohms; This is the resistance of the wire to flow of electricity.Or thinking of the difference between a small diameter hose and a largediameter hose. The range of ohms goes from zero to infinity. Zero ohmsis when it's real easy for the electricity to flow. A reading of infinityis when there is no path for the electricity to go through (a closed valve).

        Watts; This is power and is a function of amps and volts.

        DC; Direct current. Other than within the alternator, all ofthe voltages in your car is "direct" current.

        AC; Alternating current. This is what you would see in the wiringin your house. 110 volts is in reality a wave that goes from a positivevalue to a negative value. Don't play with the house wiring!!

        Primary voltage; This is the input voltage to the coil and isaround 12 volts (I'm trying to keep it simple!).

        Secondary voltage; This is the output from the coil and is MAJORvoltage. This is what ends up going to the spark plugs and will cause personaldamage if you grab it!

        Leads; These are the wires that are hooked up to the meter (I'mnot trying to be a smart ass, honest). Normally the leads are black andred.

        Alligator Clips; These are the clips that enable you to attachthe lead to something.

      • Normally when you buy one of these meters they have both functions ofreading volts and also ohms. The explanation of what they do has to bewritten separately for clarity.

        It's possible to get an analog meter or a digital meter. The digitalmeters are more costly and really aren't required for checking out mostautomotive problems.


        In order to read "volts", it has to be in a referenceto another point. Normally this reference point is ground. As an example,if you want to read the voltage of the car's battery, the negative wirefrom the gauge has to be to the negative terminal on the battery, whilethe positive wire is touching the positive terminal on the battery. Ifthe negative wire is removed, you lose the "reference" and thereis no voltage reading.

        You have to make sure that the setting on the meter is at "DC"and that the "range" is larger than the voltage that you expectto read.

        OHM METER;

        Ohms are the amount of resistance in an object to let electricity flowthrough it. With a good meter, it's possible to read the resistance fromyour left hand, through your body, to your right hand.

        How, Why, Examples, and Tips;

        Tip #1; Totally questionable wiring in car.

        When I purchased my car (64 2dr wagon), nothing worked and I had tohave the car towed home. In the process of rebuilding and modifying it,a lot of the already questionable wiring was changed.

        I didn't want to just hook up the battery and have the car go up insmoke.

        What I did was, remove all of the fuses and get a motorcycle batterywith which I could feed in electricity to the circuits that I wanted tocheck. (yes, I know the water level is low)

      Product Electronics accessory Battery Automotive battery Technology

      • I wired up a in-line fuse connector and installed a 20 amp fuse. Thisway, if there was any problem, the fuse would blow.

        I could then safely check each circuit and not fear damaging anything.It took quite some time, however there are no burnt wires to show for it!

        Example #1; Tail lights don't work.

        To check out the situation at the bulb, you have to first remove thebulb.

        1. Hook up the black lead on the meter to any metal on the body usingthe alligator clip (a screw head works great). Set the meter to ohmsand then touch the red lead to some other metal point on the body. Themeter should now read zero ohms. This confirms that the ground connectionis good.

        2. Take the red lead and touch the bulb housing. The meter should readzero ohms if the housing is grounded correctly.

        3. Now set the meter to volts. As stated before, if there areseveral setting for volt range, you have to set the meter to a range thatis larger than what you expect to read (12 volts).

        4. With the light switch turned on, take the red lead and touch thecontact inside the socket. The meter should read 12 volts.

        If you don't have 12 volts, you will have to trace the wire back andfind out where it isn't hooked up.

        If you do have 12 volts and the housing is grounded, then the connectionto the bulb has to be bad (or the bulb is bad).

        You can check the bulb by setting the meter to ohms, holding the blacklead on the bulb housing, and touching the contact. You should be ableto read the resistance of the wire in the bulb. Be carefull that you arenot reading through yourself as you hold the leads.

        Example #2; Your fuel gauge doesn't work.

        In order for a gauge to work, it needs three things. There has to bea ground reference, a solid 12 volts, and there has to the amount of resistancesent from the sending unit.

        1. Hook up the black lead to ground and verify the connection as above.

        2. Set the meter to volts and check if there is 12 volts to the gauge.

        3. Set the meter to ohms and check if the gauge has a solid ground.(One of the things that you have to be careful of when reading ohms isthat you are not reading through something. It's possible to read througha bulb, and think that you are reading ground. When in doubt, remove anyobjects that you may read through and get misleading results.)

        4. With the meter still on ohms, remove the wire from the sending unitand read the resistance (ohms). On GM sending units, an empty tank shouldread as zero ohms, a full tank should read as 90 ohms (half a tank is 45ohms).

        If you have all of above, I'd guess the gauge is bad.

        Example #3; I'm open for suggestions.
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